There has been a rapid increase in portable, cordless electric appliances of audiovisual equipment, office equipment such as personal computers, and communications equipment in recent years. Since non-aqueous electrolyte secondary batteries typically represented by high-capacity alkaline rechargeable batteries or lithium ion secondary batteries are ideally suited as a drive power source for such equipment, research is being conducted to develop a sealed non-aqueous electrolyte secondary battery of high energy density and excellent load characteristics.
One example of prior art closure assemblies for sealed batteries is shown in FIG. 19 to FIG. 21. In these figures, reference numeral 110, 120, 130 represent a metal cap, a metal spacer, and an upper metal foil, respectively. The center of the upper metal foil 130 is warped to form a dent 131, and a thin part 132 is formed by impressing on one side relative to this dent 131, as shown in FIG. 21. 140 is a dish-like insulating gasket having a bottom, and 150 is a belt-like lower metal foil. The lower metal foil 150 has a bulge 151 in its center, a slit 152 in the form of letter C surrounding the bulge 151, and punched holes 153 at both ends of the slit 152, as shown in FIG. 20. 160 is a metal case in the form of a cup which has a gas vent 161 in its center and is joined to a lead terminal that is connected to one electrode. The closure assembly for sealed batteries constructed as described above is mounted airtightly to an open end of an outer case of the battery. In the case of the closure assembly for sealed batteries described above, the upper and lower metal foils 130, 150 are electrically connected only through a welding point S in their respective centers, and the breaking strength of an uncut portion formed by the slit 152 in the lower metal foil determines the pressure at which this electrical connection is broken. Specifically, when the internal pressure of the battery which acts on the upper metal foil 130 through the punched holes 153 builds up to a predetermined value, the pressure concentrates on the warped portion 131 of the upper metal foil 130, pushing same upwards and thereby inverting the dent into a bulge as shown by a phantom line in FIG. 19. The welding point S of the lower metal foil 150 is thus pulled up, splitting apart the uncut portion of the slit 152 and thereby disconnecting the upper metal foil 130 and lower metal foil 150. The contact between the lower metal foil 150 connected to an electrode through the metal case 160 and the upper metal foil 130 connected to the metal cap 110 through the metal spacer 120 is broken whereby electric current supply is stopped. A further build-up of internal pressure leads to rupture of the thin part 132 in the upper metal foil 130, through which the gas within the battery is released to the outside.
In the event of failure, over-charging or inappropriate use of the charger and alike, pressure can build up within the battery to an excessive level due to an abnormal increase of gas generated by chemical reaction within the battery.
The battery can eventually explode or damage the equipment to which it is applied. To avert such possibility, explosion-proof features are normally provided in this and other types of batteries to release gas to the outside in case of build-up of pressure within the battery beyond some predetermined limit.
Furthermore, since there is a risk of ignition upon rapid heating of the battery in non-aqueous electrolyte secondary batteries, safety features are also provided by which power supply is stopped prior to emission of the gas in case the internal pressure of the battery exceeds a predetermined limit.
In the prior art closure assembly for sealed batteries described above, due to difficulty in controlling the machining precision of the thin part 132 in the upper metal foil 130, there exists variation in the thin part 132. As a result, the breaking pressure at the point of letting out the internal gas cannot be fixedly determined. The breaking pressure at the point when power supply is stopped is not constant either, because of the variation in dimensions of the warped portion 131 in the upper metal foil 130 and uncut portion of the slit 152 due to difficulty in controlling the machining precision. Improvement in the reliability of the explosion-proof, safety features is thus strongly desired. Furthermore, since the upper and lower metal foils 130, 150 are thin films, minute cracks that can cause leakage are inevitably formed in the welding point S where the upper and lower metal foils 130, 150 are laser-welded. Moreover, a considerable space in upward and downward directions is required in order to allow the warped portion 131 of the upper metal foil 130 to be inverted, by which the dimensions of the entire closure assembly cannot be further reduced.
An object of the present invention is to solve the aforementioned problems, i.e., to improve the reliability of safety features of sealed batteries for averting explosion. Another object of the invention is to make the closure assembly thinner while improving its leakage-proof performance.